1. Field of the Invention
The present invention generally relates to power conversion and, more particularly, to a DC to AC power conversion system of the switching type.
2. Description of the Prior Art
Various types of systems for converting direct-current (DC) power to alternating current (AC) power, hereinafter referred to as DC to AC power converters, are well known. These include converters in which the power conversion is performed by rotating machines, such as motor-generator sets, linear devices such as oscillators followed by power amplifiers, and switching type DC to AC converters. The converters, using linear or switching devices, are preferred by some users over converters with rotating machines, since the former are generally quieter, cleaner and offer superior electrical performance, especially at lower power levels. The major disadvantage of a converter using linear devices, typically an oscillator followed by a power amplifier, is its low conversion efficiency, which in practice is 50% or less. Thus, converters with linear devices are limited to applications in which only modest power levels are required, e.g., up to several KVA.
DC to AC converters of the switching type, which have been developed in recent years, are preferred, particularly at higher power levels because they exhibit higher conversion efficiency. Basically a converter of the switching type converts DC to AC by switching the input DC of alternating polarities into an input transformer, to form a square wave signal or signal of other rectangular waveform. This signal is then passed through a filter network which removes unwanted harmonics from the AC output. High efficiency is realized since the switches which are used dissipate an insignificant amount of power.
In most presently known DC to AC switching converters the switches are believed to be switched at a frequency which is equal to the desired frequency of the AC output. The typical AC output frequency ranges between about 50 Hz to 400 Hz, e.g., 50, 60, 400 or slightly beyond. For example, in a prior art switching converter, operated to produce AC output at 50 Hz, typically the switches are switched at a 50 Hz rate. That is, the switches go through their complete On-Off cycle in 20 ms and the output voltage is driven to its peak value every 10 ms. Due to the low switching frequency, which is typically the same as that of the desired AC output power frequency, the filter has to filter out harmonics of the AC output frequency which is quite low. Consequently, the filter components, such as the inductors and capacitors are large and heavy.
Also, in a prior art switching converter if a regulator is included, which is generally the case, the regulator response time (speed) is limited by the low switching frequency. The regulator response time is commonly on the order of hundreds of milliseconds. Consequently, the regulation of the output waveform does not take place within one cycle but rather over a large number of cycles. Due to these factors presently known switching converters are typically quite large and heavy and produce AC output power of only moderate quality.